Okay so imagine your phone is like a toy car and it needs something called a road to drive on. This road is like a big network of telephone wires.
The people who are responsible for building and maintaining these telephone wires are called telephone companies. They make sure that your toy car can go anywhere it wants to go, and it won't crash into anything along the way.
But a long time ago, there used to be only one big telephone company that ran everything. They were in charge of building the roads, maintaining them, and making sure that everyone had access to telephones.
However, the government decided that it wasn't fair for one company to have all the power. So they made a big decision and split up the company into smaller companies.
The smaller companies were called Regional Bell Operating Companies (or RBOCs for short). Each RBOC was responsible for building and maintaining the telephone wires in a specific region of the country.
Think of it like having lots of smaller toy car roads instead of one big one. For example, if you live in New York, the RBOC responsible for your area is called Verizon. They make sure that all the phone lines in New York are working properly and that people can make calls without any problems.
So RBOCs are basically smaller companies that do the same job as the big telephone company used to do, but they only work in specific regions.